1. Field of the Invention
The invention relates to a process for fabricating a photomask, and more particularly, to a process for repairing phase-shifting photomasks.
2. Description of the Prior Art
For next generation and future technology, mask making is one of the bottlenecks. Phase shift masks take advantage of the interference effect in a coherent or partially coherent imaging system to reduce the spatial frequency of a given object, to enhance its edge contrast, or both. It is possible to control locally the type of interference, destructive or constructive, at critical locations in a design by adding an additional patterned layer of transmitting material on the mask. This technique results in a combination of higher resolution, larger exposure latitude, and larger depth-of-focus. In phase shift lithography, a transparent coating is placed over a transparent area. The light waves passing through the coated region are delayed 180xc2x0 out of phase with the light waves passing through the uncoated region. At the edge of a phase-shifted area, the light waves from the phase-shifted and clear areas will cancel out producing a more sharply defined interface.
An attenuated phase shifting mask (APSM) includes an attenuator which is a metallic-like light absorbing film such as molybdenum silicide oxynitride (MoSiON) or chromium oxynitride (CrON) which allows 5-15% light transmittance. The partial transmittance of the light waves through the attenuator allows for phase shifted light to be produced.
As critical dimension decreases, defects can occur in the making of masks, necessitating repairs. The repair performance of a mask is a key issue in controlling critical dimension and process windows after lithography. Repair machines have limits to their accuracy and capability for repairing advanced masks. One current method of mask repair is by focused ion beam (FIB). For example, to achieve a smaller critical dimension size for a larger pattern such as a contact hole, a clear area is deposited. To achieve a larger critical dimension size for a smaller pattern, the dark area is etched. FIG. 1 illustrates a top view of a photomask pattern 10. 20 and 30 are types of repairs made on the uncoated area.
This FIB method can result in problems. The deposition method can result in over-deposition to produce blind contact holes or reduced critical dimension size. Or the repairs may have damaged the uncoated area. Thus, defects may need to be repaired several times. This will reduce throughput and may result in damage to the quartz substrate if several etching processes are required to recover the critical dimension size of the defect. The current deposition/etching repair methods are hard to control for smaller featured patterns. The copy method of the current FIB repair machine cannot work for abnormal defects in which the image of the FIB from the repair machine is the same as the normal pattern, but the aerial image""s measurement from a micro-lithography simulation microscope (MSM) is totally out of specification for the critical dimension size. That is, although the contact hole size is correct, the abnormal holes may not get enough light intensity compared to normal holes because of residues on uncoated areas or non-uniform dry etching on uncoated areas, for example.
U.S. Pat. No. 5,443,931 to Watanabe discloses a method to repair defects in a mask where resist is used to fill in dent defects and to form sloped sidewalls around bump defects. U.S. Pat. No. 5,272,024 to Lin discloses a PSM having three layers of phase-shifting material. Repairs are made by etching away one or more of the phase-shifting layers. U.S. Pat. No. 5,506,080 to Adair et al teaches a method of observing defects in the resist layer formed on a mask. The defects are repaired in the resist layer, then the repaired pattern is transferred to the mask layer. U.S. Pat. No. 6,016,357 to Neary et al uses aerial image measurement to determine the presence of defects. Patches are added to repair the defects. Patches could be corners or anchors of patch material in clear areas to provide optical proximity correction (OPC).
Accordingly, it is a principal object of the present invention to provide an effective and very manufacturable process for repairing a photomask.
Another object of the present invention is to provide a process of repairing a phase-shifting photomask.
Yet another object of the present invention is to provide a process of repairing a binary photomask.
Another object of the present invention is to provide a process of repairing an attenuated phase-shifting photomask wherein the critical dimension variation can be predicted during repair.
A further object of the present invention is to provide a process of repairing a photomask wherein distorted patterns are repaired.
A still further object of the present invention is to provide a process of repairing a photomask wherein substrate damage during repair is minimized.
A still further object of the present invention is to provide a process of repairing an attenuated phase-shifting photomask wherein quartz-damaged patterns are recovered by aerial light""s contribution of optical proximity correction scattering bar pattern.
Yet another object of the present invention is to provide a standardized process of repairing an attenuated phase-shifting photomask for critical dimension control.
In accordance with the objects of this invention a new process for repairing an attenuated phase-shifting photomask is achieved. A contact hole pattern is provided on an attenuating phase-shifting photomask. An aerial image is obtained of the contact hole pattern. The critical dimension of the contact hole pattern is predicted from the intensity of the aerial image. Thereafter, the critical dimension is adjusted by forming non-printable optical proximity or scattering bar correction patterns around abnormal defects in the contact hole pattern on the attenuated phase-shifting photomask. The non-printable correction patterns enhance or cancel light intensity to correct the adnormal defects.